If you have problems sending e-mails or if your recipients inform you that no more e-mails will be received from you, it may be that the IP of your server has landed on a so-called blacklist.
A blacklist is a list of IPs from which SPAM or even hack attacks have been registered and therefore get into such a revocation list. Such blacklists / blacklists are used by many providers to prevent SPAM as early as possible. So if the IP of an outgoing e-mail server is in one or more such lists, many incoming mail servers do not even accept the communication from that server. This is a common and usually meaningful approach. Even with hostNET so-called blacklists are used.
But sometimes you end up in such a list and may not know it. And most of all, why that might be the case.
If you or the IP of your server is on such a list, can be easily found out with this guide. To find the reason is always a second step.
Check your own mail server IP
The check is always about the IP of the mail server. You can check your IP, if it is blocked here: https://www.dnsbl.info/dnsbl-database-check.php
Go to the website, enter your IP and check.